Support Markdown in Add-on Listing Fields

[chrstinalin]

Fixes: mozilla/addons#15145

Description

Replaces HTML syntax support with Markdown support in the description, developer_comments, eula, and privacy_policy fields of Addon. Existing HTML in those fields will remain functional until the field is edited. No new HTML will be functional.

Only the markdown-equivalent of currently allowed HTML attributes are allowed. abbr uses the PHP Markdown Extra syntax.

DevHub:

Frontend:

Testing

New Add-ons

1. Create a listed add-on.
2. Under 'submit/details', description should have the tooltip and support markdown.

Existing Add-ons

1. Navigate to an add-on that already has HTML in its description. The HTML should still be functional. The HTML is still rendered on the frontend listing.
2. Under the 'edit' page, the description and developer comments should support markdown syntax.*
3. Under the 'Manage Authors & License' page, the EULA and Privacy Policy fields should support markdown syntax.*

* Any user-inputted HTML saved at this point is rendered as plaintext. Any allowed markdown in the field is rendered as expected in the frontend. Disallowed markdown is stripped from the result and not visible at all (this is to avoid showing the user the markdown formatted into HTML).

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.

[chrstinalin]

There should probably be a more informative way to explain what syntax or Markdown is supported, possibly similar to how GitHub explains it via the 'Markdown is Supported' button by navigating to a dedicated page.

[KevinMind]

Looks promising. A few comments before testing.

[KevinMind]

Please add testing steps and I can verify the PR.

[KevinMind]

@chrstinalin what should I expect to happen if I enter invalid or disallowed markdown syntax? Can you add that to the test scenario. Additionally can you clarify what "functional" means? I think you mean the syntax should be "rendered" on the page right?

[KevinMind]

Can you add a test of mixed html and markdown, simulating someone editing an existing html string and adding markdown to it. We would expect the html to be stringified and the markdown to be formatted to html right?

[chrstinalin]

test_markdown does have a case of mixed HTML and markdown, and yeah -- the HTML is stringified, the markdown is formatted.